The present invention relates to spinning of pitch and more particularly to a pitch melt to be used in production of carbon fibers.
Carbon fibers have been increasingly used in various applications because they are light and have high electrical conductivity and heat resistance. In particular, carbon fibers made from pitch fibers are widely used because over those fibers made from other carbonaceous materials, the yield at the carbonization step is high, and the modulus of elasticity of fibers is high.
If, however, cracks are formed in pitch fibers, carbon fibers obtained by calcination of the pitch fibers will suffer from development of cracks therein, thereby resulting in a reduction of mechanical strength thereof. Thus, in order to inhibit the development of cracks in pitch fibers to increase the mechanical strength of the final carbon fibers, various spinning methods for production of pitch fibers have been proposed.
For example, Japanese Patent Application Laid-Open Nos. 88909/1984 and 12919/1986 disclose a pitch spinning method in which pitch is passed through a filler layer provided above a spinning nozzle and, thereafter, it is passed through the spinning nozzle. Experiments, however, have revealed that in accordance with the above method, the development of crackes can be prevented to a certain extent, but not sufficiently and furthermore, if spinning is continued for a long period of time, spinning unevenness is produced.
In melt spinning of pitch containing optically anisotropic components, pitch molecules are orientated in a fiber axis direction. It has been found that even if the same pitch is used, the orientation in cross section of a carbon layer varies with spinning conditions and according to microscopic observation of the cross section vertical to the fiber axis, it can be divided into three types: radial type in which the cross sectional orientation is in a radial form, onion type in which it is in a concentric form, and random type in which there is no regular orientation.
In the case of pitch fibers having the radial type cross section, when they are subjected to carbonization or graphitization treatment after the spinning step, cracks will develop in the surface thereof in a direction of the fiber axis and seriously decrease the strength of the final carbon fibers.
It is therefore generally said that pitch fibers having the random or onion type cross section are preferred for production of carbon fibers. For this reason, an endeavor has been made to find spinning conditions which produce pitch fibers having a cross section of either the random type or the onion type.
For example, pitch fibers having the onion type cross section can be relatively easily produced only when a single fiber is spun, but they are very difficult to produce when a number of fibers are spun by the use of one nozzle unit on a commercial scale. Thus, in order to prevent cracks in pitch fibers at the step of spinning on a commercial scale, various melt spinning methods to produce the onion type cross section have been proposed.
For example, a method in which various fillers are charged in an upstream space to a hole or opening of a nozzle (hereinafter referred to as a "nozzle hole") (see, for example, Japanese Patent Application Laid-Open Nos. 88909/1984, 259609/1985 and 186520/1986) and a method using a nozzle the inside surface of which is shaped in a complicated form (see, for example, Japanese Patent Application Laid-Open Nos. 163422/1984, 168127/1984 and 252723/1985) are known.
The former method, however, has disadvantages in that since the filler is not packed uniformly to all of the nozzle holes, there cannot be obtained pitch fibers having a uniform diameter and, moreover, since the space in which the filler is packed is usually very small and there are a number of nozzle holes, charging of the filler and cleaning of the space are too complicated.
The latter method also has disadvantages in that the nozzle is difficult to produce because there are a number of nozzle holes and the diameter of the nozzle hole is too small, and cleaning of the nozzle is too difficult.